Projection screens are well known in the art which are employed for picture display. Most screens as they are traditionally known have a material on the surface thereof for the purpose of dispersing light in a multitude of directions which is incident upon the screen from the projector.
In situations where the efficiency of the screen is important, attempts have been made to direct light incident upon the screen from the projector into a predetermined viewing area. For example in my prior patent, Ser. No. 178,990 which was filed on Sept. 9, 1971 and is entitled "Lenticular Projection Screen" now U.S. Pat. No. 3,712,708, a lenticular projection screen is disclosed which consists of a plurality of overlapping spherical concave lenses formed in reflective material. The lenses and the screen are constructed so that the projected light rays are reflected to predetermined viewing areas in front of the screen.
When screens are made of a reflective material it is of course obvious that reflections from sources other than the projector become a problem. In the past people have overcome this problem by using the screen in a dark room. Attempts have been made to use such screens in lighted areas by putting light absorbing elements in predetermined locations with respect to the reflective material in the screen. These attempts have been somewhat successful but have added additional annoyance to the use of the screen and/or additional cost and encumberance in the manufacture and use of the screen. Notwithstanding these efforts to overcome the problems of unwanted reflections, difficulties are still encountered when using these screens.
Even in a dark movie theater, people open the door to come in and go out and ushers use flashlights which create difficulty with reflections therefrom. The use of light absorbing material while having a limited beneficial affect still does not completely overcome the problem of unwanted reflections.
On Dec. 14, 1926, U.S. Pat. No. 1,610,423 issued to A. J. Cawley which was entitled "Daylight Projecting System." In FIG. 4 of this patent a daylight projection screen is disclosed in which a transparent material is shaped to have prism like ridges on the back thereof for the purpose of reflecting light which is incident thereon from a predetermined angle. Cawley teaches that the critical angle of the material should be as close to 45.degree. as possible and that the angle of the sides of the prism like portions should be inclined with respect to the plane of the material at 45.degree.. The reason for this is to insure that only light incident upon the screen from a single direction will be reflected back towards the audience. All other light incident upon the screen will strike one side of the prism or the other at an angle less than 45.degree. with respect to a normal to the surface thereof and pass therethrough rather than be reflected.
The Cawley system is an interesting curiousity but is of no practical significance since all of the light is focused back in a single direction so that the audience must be located directly in front of the screen. If the screen is moved by an angle of say 1.degree. with respect to the incoming projected light, the image reflected back to the audience will be lost because the largest portion of the light incident upon the screen from the projector will pass therethrough rather than being reflected back. Further, if the audience is not sitting at the precise angular relationship with respect to the screen, they will again see very little because the screen of Cawley will project light back parallel to the rays incident thereof and provide no dispersive angular affect of reflection.
In my prior U.S. Pat. No. 3,782,805, I disclosed a front projection screen made from a sheet of light transmitting material and having a predetermined critical angle of internal reflection less than 45.degree.. That front projection screen, as the one disclosed herein, had a plurality of parallel ridges on the front and the back thereof with specific relationships (as does the screen of the instant invention) between the ridges on the backside thereof and the critical angle of internal reflection.
In that patent, I taught that the thickness of the screen determined the degree of vertical dispersion and further taught that with a zero thickness, maximum dispersion would be achieved and with a thickness equal to the focal length of the ridges on the front surface, no dispersion would be achieved. In that patent, I taught further that the optimum thickness for mechanical stability would be between one and one and half times the focal length of the front ridges.
I have found that a screen of such thickness will not produce double images as taught in my patent. I have further found that unwanted dead spots in the viewing area does result. I have further found that if the screen is constructed of a thickness for optimum dispersion in the vertical plane, the screen cannot be tilted in the vertical plane with respect to the projected light without losing some reflection in the viewing area. As a result of this, the vertical location of the projector with respect to the screen becomes critical and, further, often requires the location of the projector at a vertical location which interferes with the viewing area.
Therefore, it is an object of this invention to provide a new and improved front projection screen.
It is a further object of this invention to provide a front projection screen which efficiently uses the light provided by the projector, does not reflect undesirable images to the audience and is viewable from positions other than directly in front of the screen.
It is still another object of this invention to provide a front projection screen which may be employed in daylight with a minimum of reflection without the use of light absorbing members affixed thereto.
It is yet another object of this invention to provide a front projection screen which can be tilted in the vertical plane with respect to a projector without affecting reflection.